This invention relates to an ultrasonic transducer for use in the acquisition of a cross-sectional image of a living body for a medical diagnosis, and in the measurement of the living body, or in a non-destructive examination, and also relates to a manufacturing method of the ultrasonic transducer.
An ultrasonic transducer is designed to transmit an ultrasonic pulse to an object and to receive a reflected signal (echo) from the object. The object may be a living body. It is possible through an analysis of the echo to obtain information with respect to the object.
The construction of the ultrasonic transducer is described for example in "Hand Book of Medical Ultrasonic Equipments" (Nippon Electronic Mechanical Industries Association; Corona Publishing Co., Ltd., 1985, 4/20, pp186-190).
The ultrasonic transducer fundamentally comprises a piezoelectric body, an acoustic matching layer and a backing material. The piezoelectric body is provided on the opposite surfaces thereof with electrodes and is designed to convert an ultrasonic pulse and echo respectively to a voltage pulse. The body may be a piezoelectric ceramic formed in a board. The acoustic matching layer is designed to reduce a transmission loss in the ultrasonic pulse between the object such as the living body and the transducer. The backing material is designed to shorten the wave-shape of the pulse to improve the resolution of the transducer. In addition to the above members, an acoustic lens for converging the pulse to the object may be disposed in front of the acoustic matching layer.
The ultrasonic transducer can be operated as follows. A hundred to several hundreds volts of a driving voltage pulse is applied from a pulser to the electrodes. The driving voltage pulse can suddenly deform the piezoelectric body due to a reverse piezoelectric effect. The deformation can excite the ultrasonic pulse which is then emitted via the acoustic matching layer and the acoustic lens.
The ultrasonic pulse emitted is then reflected by the object. The reflected pulse from the object re-enters via the acoustic lens and the acoustic matching layer into the piezoelectric body to oscillate the latter. The object may be an interface between tissues of living body when the transducer is used for a medical purpose, or a discontinuous portion such as a flaw inside a measuring object when the transducer is employed for a non-destructive examination. The mechanical oscillation of the body generated by the re-entered pulse can be converted by way of piezoelectric effect into an electric signal which is then transmitted to a monitoring device to produce an image.
The ultrasonic pulse is generally converged into an ultrasonic beam to be scanned over an object to produce an object image.
Two systems, for example, are known for converging the ultrasonic pulse into ultrasonic beam. In a first system, the front surface of the body is curved to converge the beam to a geometric center of the curvature, or an acoustic lens is disposed in front of the body through which the beam is converged. In a second system, the transducer is formed of a plurality of very small transducer elements which are driven with a phase shift between them so as to converge the beam.
The method for scanning the ultrasonic beam may be following two systems.
In a first system, an ultrasonic beam can be scanned by actuating one or more transducers, while changing the angle and position of each transducer. This system is called a mechanical scanning system.
In a second system, as mentioned above in reference to the converging method of ultrasonic beam, an transducer is formed of a plurality of very small transducer elements. The beam can be scanned by selectively driving those elements individually or in group with phase differences between them. This transducer is called an array type or electronic scanning type ultrasonic transducer.
The electronic scanning type ultrasonic transducer is generally composed of several tens to several hundreds elements, each having a very small width of 0.5 to 1.5 mm, or less than 0.5 mm, which are arranged on a flat or curved surface. Each element is also fundamentally consisted of a piezoelectric body with electrodes on the opposite surfaces thereof, an acoustic matching layer and a backing material. Generally, the piezoelectric body in each element is further divided into 2 to 3 sub-bodies each being rod-shaped for example. Division of the piezoelectric body into sub-bodies can suppress the generation of oscillation modes other than the thickness mode. Since those oscillation modes do not contribute to the transmitting and receiving the ultrasonic waves, they are required to be suppressed. Further, the division into sub-bodies can minimize the loss in transmission of ultrasonic pulses, and improve the electromechanical coupling factor and piezoelectric constant of the body.
In the ultrasonic transducer of mechanical scanning type, the same effects can also be obtained by dividing the body into sub-bodies.
In recent years, a composite piezoelectric body has been increasingly employed in place of a single piezoelectric body, which combines a sub-body or a rod-shaped body and a resin. The employment of the composite body can further minimize the loss in transmission of the ultrasonic pulses which might be caused by a difference in acoustic impedance between the piezoelectric body and the living body, and suppress useless resonance modes, and improve the electromechanical coupling factor and piezoelectric constant of the ultrasonic body.
The composite body can be employed in an ultrasonic transducer of both the mechanical scanning system and electronic scanning type.
The composite piezoelectric body employing a sub-body or a rod-shaped body can be manufactured by dicing a single piezoelectric body. This method is explained for example in Japanese Patent Unexamined Publication No. S60-85699 (hereinafter, referred to as a prior art 1). The dicing method involves the following procedures. First of all, a bulk piezoelectric body consisting of for example lead zirconate titanate (PZT) is adhered to a substrate with an adhesive. Then, the bulk body is diced into a stripe or matrix pattern on the substrate with a dicing device. The groove portion formed as a result of the dicing is filled with a resin such as epoxy resin or urethane resin, and then, the resin is allowed to cure. After the curing, the bulk body diced in this manner is removed from the substrate, thereby obtaining a desired composite body. This method includes two different procedures for obtaining an ultimate composite body. One of the procedures involves the steps of dicing a piezoelectric body, filling the diced groove with a resin, and removing the body from the substrate after the resin has been sufficiently cured, thus obtaining a desired composite body. The other procedure involves the steps of dicing incompletely a piezoelectric body, filling the diced groove portion with a resin, removing the body from the substrate after the resin has been sufficiently cured, and grinding or slicing the body thereby to obtain a desired composite body.
The composite body employing rod-shaped piezoelectric body can be obtained also by producing a fine structure of piezoelectric body with a micromachining technology. This method is described for example in "Jpn. J. Appl. Phys."; Vol. 36(1997), pp.6062-6064 (hereinafter, referred to as a prior art 2). The prior art 2 discloses a method wherein a deep X-ray lithography and a resin molding are combined to obtain a rod-shaped bodies of a high aspect ratio.
More specifically, a resist film having a thickness of 400 .mu.m and consisting of MMA (methyl methacrylate)/MAA (methacrylic acid) copolymer is coated on a substrate.
Then, a synchrotron radiation is irradiated through a mask onto the resist film, followed by development of the film, thereby obtaining a resist structure having a plurality of openings. Thereafter, using the resist structure as a resin mold, a PZT slurry is poured into the openings. The PZT slurry is consisted of a PZT powder, a binder and water.
Further, the PZT slurry is allowed to dry and cure at room temperature to obtain a PZT green body. Then, only the resin mold is removed with an oxygen plasma to leave the PZT green body. The PZT green body thus left is next subjected to a calcinating treatment (removal of the binder) at 500.degree. C., and then to a sintering treatment at 1,200.degree. C. As a result of the sintering, a PZT rod array can be obtained, of which each rod has 20 .mu.m in diameter and 140 .mu.m in height.
Then, the space between the PZT rods is filled with epoxy resin using vacuum impregnation, followed by curing the resin. After curing, the upper and bottom surfaces of the rod array are polished to expose the opposite end faces of the PZT rod, thereby flattening both the surfaces of the array. A gold electrode is deposited by sputtering on the flattened surfaces of the rod array. Then, the array is subjected to poling with a voltage applied onto the electrodes while keeping the array immersed in an oil bath. Thus, a composite piezoelectric body can be provided with a piezoelectric property. The composite body thus obtained can have a frequency constant of not more than 700 kHz.multidot.m, and be manufactured in a mini and thin shape.
However, an ultrasonic transducer employing the composite piezoelectric body is accompanied with problems that the wiring to each piezoelectric body is complicated, and the electrodes can easily have breaks during the operation of the transducer.
In order to wire to each sub-piezoelectric body, a wiring is connected either to the electrode of each sub-body, or to a continuous electrode formed over the sub-bodies. On the other hand, in order to wire to each rod-shaped piezoelectric body, a wiring is connected to a continuous electrode formed on a surface including both the end faces of each rod-shaped body and the exposed surface of resin formed between the rods.
The wiring to each sub-body involves a complicated work. As a result, the construction of the resultant ultrasonic transducer as well as the manufacturing steps are complicated. Further, the continuous electrodes occasionally have breaks during the operation of the transducer thereby causing a disconnection of wiring. More specifically, only the sub-bodies or the rod-shaped bodies can oscillate independently, while other members such as the resin oscillate following the movements of the bodies. Therefore, the continuous electrodes may have breaks at the boundary portions between the independently oscillating bodies and the dependently oscillating members.